The McMurray oil sands of Alberta constitute one of the largest deposits of hydrocarbons in the world. The oil sands are first mined at a mine site and then transported to an extraction plant in order to extract the bitumen. In recent years the preferred mode of transport of mined oil sands has been by way of a slurry pipeline. The oil sand is mixed with water to form a slurry that is capable of being pumped down a pipeline to the extraction plant.
One needs to provide a suitable means for slurrying the oil sand with water and entraining air to produce a slurry that is suitable for pumping down the pipeline. The as-mined oil sand contains a variety of lumps including rocks, clay and oil sand lumps. Therefore a mixer means is required that not only slurries the oil sand but also ensures that oversize lumps that are unsuitable for pumping and feeding into the pipeline are rejected. A typical aqueous slurry comprises the following: bitumen froth, sand, smaller lumps of oil sand, clay and/or rocks (between 0 and 2 inches in diameter) and larger lumps of oil sand, clay and/or rock (between 2 and 4 inches in diameter).
In U.S. Pat. No. 5039227, issued to Leung et al and assigned to the owners of the present application, one mixer circuit for this purpose has been disclosed.
In the Leung et al mixer circuit, an oil sand stream is dropped from the end of a conveyor into a mixer tank. The mixer tank is open-topped, has a cylindrical body and conical bottom and forms a central bottom outlet. A swirling vortex of slurry is maintained in the tank and the incoming oil sand and added water is fed into it. Slurry leaves the tank through the bottom outlet, is screened using vibrating screens to reject oversize, and is temporarily collected in an underlying pump box. Some of the slurry in the pump box is withdrawn and pumped back through a return line to be introduced tangentially into the mixer tank to form the swirling vortex. The balance of slurry in the pump box is withdrawn and pumped into the pipeline.
In a co-pending application, a second-generation mixer circuit in the form of a vertically oriented stack of components, functions to slurry the oil sand with water. The oil sand is initially dropped from the end of a conveyor and is contacted in mid-air with a stream of water. The mixture drops into a downwardly slanted trough and the water and oil sand mixes as they move turbulently through the open-ended trough. The slurry is deflected as it leaves the trough and is spread in the form of a thin sheet on an apron. It is then fed over screens to reject oversize lumps. The screened slurry drops into a pump box where it is temporarily retained. The rejected lumps are comminuted in an impactor positioned at the end of the screens. The comminuted oil sand is screened to remove remaining oversize lumps and the screened comminuted oil sands are delivered into the pump box. The slurry in the pump box is withdrawn and pumped into the pipeline.
Both of the prior art mixer circuits routinely produce a slurry that contains lumps ranging from 0 to 4 inches in diameter. Before the slurry is pumped to the pipeline, it is temporarily stored in a pump box. The pump box is restricted to a certain volume because if the volume of retained slurry is too great, settling of the sand and lumps will occur. As a result, the residence time of the slurry in the pump box is relatively short (in the order of 1 minute) and the slurry is quickly pumped from the pump box to the pipeline.
As the slurry travels down the pipeline, slurry conditioning or digestion takes place. Adequate conditioning is critical for good bitumen recovery in a downstream separation vessel and is especially important when extracting bitumen from low grade oil sand. Basically what conditioning means is that the larger oil sand lumps are ablated into smaller lumps and bitumen flecks coalesce and coat or attach to air bubbles. The lumps need to be dispersed in water to promote the release of oil droplets and the attachment of air. Conditioning also benefits from turbulent pipeline flow and is dependent upon the length of the pipeline, hence, the length of time that the slurry resides in the pipeline before reaching the separation vessel. The larger the oil sand lumps, the more time required to digest or ablate these lumps to release the bitumen flecks. Therefore if a slurry is routinely produced that contains large lumps, there will be a need for long pipelines or residence time.
An ideal slurry for fast conditioning (i. under 10 minutes) would be one that consists of lumps that are less than 2 inches in diameter. But producing such a slurry is impractical due to limitations of the prior art mixer circuits. For example, in the second-generation mixer circuit, slurry routinely contains lumps that are 2 to 4 inches in diameter. This is as a result of limitations in the mixer circuit with respect to the screening process. These circuits must accommodate large throughputs of oil sand. Therefore, the screen openings must be considerably larger than 2 inches, hence, larger lumps (i.e. 2 to 4 inches in diameter) are introduced into the pipeline. This means that the pipeline has to be a certain length to ensure sufficient residence time of such a slurry (preferably a minimum of 4 km to give a residence time of approximately 12 to 15 minutes) for proper conditioning to occur.
There may be times, however, when it is unnecessary to have such a long pipeline. But if the pipeline is too short, the residence time of the slurry in the pipeline will be too short for proper conditioning of the slurry to occur. This will result in a decrease in bitumen recovery. However, a pump box can be designed whereby the harder to digest 2 to 4 inch lumps are segregated from the rest of the slurry and are directed to an impactor where they are comminuted to small lumps. Therefore the length of the pipeline becomes less critical.